Inheritance: from code reuse to reasoning reuse

In the object-oriented approach, a designer can, given an existing base class, use inheritance to build a derived class that extends, or that slightly differs from the base class, but in order to exploit the full potential of inheritance to build systems incrementally, the designer must also be able to reason about the derived class incrementally. This paper presents a specification notation and verification procedure that allows such incremental reasoning out; the approach makes important use of the concrete specification of a class, in addition to the usual abstract specification. The reasoning reuse that the approach enables is illustrated by applying it to a simple example.